Right click My Computer, select Properties and make sure both have a unique name and are a member of the same workgroup.
Next, make sure both computers have a user with the same name and password. If you don't use passwords, you should.
Right click the drive you want to share, and make a new one. Don't use the wizards, just click that you understand the risks twice (on XP), specify a name and check the box.
Turn off the firewalls (temporary) to make sure, and browse the network. If you still cannot see the other computer, choose "map network drive" in the explorer, and enter \\computername\sharename as the target.